1. Field of the Invention
This invention relates generally to power control in a wireless communication device transmitter, and, more particularly, to a system for closed power control feedback loop using a linear or a non-linear amplifier.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
In a typical global system for mobile communications (GSM) mobile communication system, a gaussian minimum shift keying (xe2x80x9cGMSKxe2x80x9d) modulation scheme supplies a very clean phase modulated (xe2x80x9cPMxe2x80x9d) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used thereby allowing efficient modulation of the phase modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Other transmission standards, however, are being developed in which both a PM signal and an amplitude modulated (xe2x80x9cAMxe2x80x9d) signal are transmitted. Standards such as these increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, existing GSM modulation schemes are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the life of the battery or other power source.
This condition is further complicated because transmitters typically employed in GSM communication systems transmit in bursts and must be able to control the ramp-up of the transmit power as well as have a high degree of control over the output power level over a wide power range. This power control is typically performed using a feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the control input of the power amplifier.
When attempting to include a PM component and an AM component in a GSM type modulation system, the power control loop will tend to fight against the amplitude variations present in the signal while attempting to maintain the desired output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal.
In such transmission signals containing both PM and AM components, the output power can be controlled by applying a predetermined control voltage to the power amplifier. Unfortunately, this requires the use of a highly linear, and therefore very inefficient, power amplifier. In non-burst transmission systems the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Another known method to control the output power is to xe2x80x9cpredistortxe2x80x9d the modulated signal in such a way that the power control loop will cancel the effect of the predistortion. In such a method, the amplitude information is passed through a transfer function that is the inverse of the power control loop transfer function. Unfortunately, these methods are costly and inefficient.
In those transmission standards where both a PM signal and an AM signal are sent to a power amplifier, unless the power amplifier is very linear, it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier.
Further still, in some communication systems, the output power is controlled by a signal from the base station with which the portable transceiver is communicating. Typically, in such an arrangement, the base station simply sends a signal to the portable transceiver instructing the portable transceiver to increase or decrease power. In such a system, there is no specific power requirement, just the command to either increase or decrease power output. Therefore, within such a portable transceiver, there is no need for closed loop power control; an open loop power control is sufficient. In open loop power control, a control signal within the mobile unit requests a particular power level without any feedback as to whether the value is achieved. The control characteristic need only be monotonic, but not particularly accurate, because the base station effectively provides feedback by continually updating its increase/decrease requests. In other transmission systems, such as GSM (which employs only PM) or GSM-EDGE (which employs both AM and PM), the power output of the portable transceiver must be more closely controlled. For example, instead of a relative power measurement as mentioned above, it may be desirable to output a specific power level. In such systems, a closed loop power control system is necessary to achieve the close output power tolerances. As mentioned above, in a closed loop power control system a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the input of the power amplifier, thus providing a higher degree of accuracy in the output power level.
Furthermore, there is a need in the industry for a transmission technique that can be used to transmit a signal having both a PM component and an AM component, while retaining the benefits of using a non-linear, and therefore highly efficient, power amplifier and the ability to supply the PM modulated signal to the power amplifier directly from an oscillator, while employing a closed loop power control system. Also, there is a need in the industry for a transmission technique in which a closed loop power control system is applied to a linear power amplifier in which a signal having both a PM component and an AM component is amplified.
The invention provides a system for power control using a closed power control feedback loop in applications using either a linear or a non-linear power amplifier. In one aspect, the invention may be conceptualized as a method for controlling output power from an amplifier using a closed power control feedback loop, comprising the steps of supplying a first modulated signal to a power amplifier, detecting a portion of an output of the power amplifier using a power detector in a closed power control feedback loop, adjusting the output power of the power amplifier based upon the detected portion of the output of the power amplifier and a reference signal, and injecting a second modulated signal into the feedback loop using a variable gain element.
Architecturally, the invention can be conceptualized as a system for controlling output power from an amplifier using a closed power control feedback loop, comprising a power amplifier configured to receive a first modulated signal, a power detector located in a closed power control feedback loop and configured to detect a portion of an output of the power amplifier, a comparator configured to develop an error signal used to adjust the output power of the power amplifier based upon the difference between the detected portion of the output of the power amplifier and a reference signal, and a variable gain amplifier configured to inject a second modulated signal into the feedback loop.
The invention allows the injection of an AM signal into the power control feedback loop of a non-linear power amplifier, and also allows closed loop feedback control of a linear power amplifier in which both a PM signal and an AM signal are amplified. By injecting an inverted version of the desired AM signal, the closed power control loop of the invention can control power in both a system in which only a PM component of a signal is supplied to the input of a non-linear amplifier and a system in which a modulated signal including both a PM component and an AM component are supplied to a linear amplifier.